The present invention relates to a photosensitive film usable more particularly in microlithography for producing electronic components, such as integrated circuits. Microlithography is a widely used technique in the field of the production of electronic components, where it is used for producing on a substrate designs having lines of approximately 1 micron or submicron lines.
In such microlithography techniques, on the substrate is generally deposited a photosensitive film. On said film is then formed by irradiation, designs which are then revealed by using a solvent for dissolving either the film zones which have been exposed to irradiation, or the film zones which have not been exposed to irradiation. In this way, openings are formed in the film covering the substrate and it is then possible to carry out treatments, such as etching or ion implantation on the thus revealed substrate zones.
During irradiation, there is a chemical reaction in the film, which can either lead to increased solubility of the film due to depolymerization, or to a chemical modification of the film. Conversely it can lead to an insolubility of the film in certain solvents resulting from a crosslinking or chemical modification. In addition, by choosing an appropriate treatment or solvent, it is possible to eliminate the film either at the locations where it has been irradiated, or at the locations where it has not been irradiated.
In the first case where the irradiated zones are eliminated, the photosensitive film is of the positive type, whereas in the second case where the non-irradiated zones are eliminated, the photosensitive film is of the negative type. In all cases, the photosensitive film must fulfil a double function. Firsty, it must permit a good degree of resolution, i.e. a good definition of the designs constituted e.g. by micron or submicron lines. Secondly, the non-eliminated zones of the film must be able to resist the following treatments performed for producing integrated circuits and must in particular resist dry etching by gaseous plasma. Generally, it is difficult to perfectly fulfil these two functions by means of the same photosensitive film layer. Thus, a good resolution of the design necessitates a very fine layer, generally having a thickness less than 0.5 .mu.m, whereas a good resistance to dry etching requires the presence of a thicker layer, generally having a thickness exceeding 1 .mu.m. Hitherto, to meet these two requirements, use has been made of multilayer systems. Such systems are more particularly described in the article by B. J. Lin, "Multilayer Resist Systems", appearing in the work published by L. F. Thompson, C. G. Willson and M. J. Bowden entitled "Introduction to Microlithography", ACS Symposium Series (Washington DC), 1983, pp. 287 to 350.
FIGS. 1 to 4 show a multilayer system of this type and illustrate its use for masking certain zones of a substrate.
In FIG. 1, it is possible to see that the substrate 1, e.g. formed by a silicon substrate, is covered by a multilayer system 3 formed by three layers 3a, 3b and 3c. The first layer 3a, directly deposited on substrate 1 is a thick resin layer, e.g. having a thickness of approximately 2 .mu.m and it is called the levelling layer, because its function is to level out topographic variations on the substrate. The second layer 3b is an intermediate layer formed from a material resisting reactive gaseous plasmas and e.g. SiO.sub.2, Si, Si.sub.3 N.sub.4 or Al. The third layer 3c is a thin photosensitive resin layer with a thickness of 0.2 to 0.5 .mu.m, which is used for the inscription of the patterns by irradiation with a good resolution.
In order to inscribe patterns on a substrate using a system of this type, the substrate coated at the desired locations, like that shown in FIG. 1 is irradiated, as shown by the arrows, followed by the elimination of the zones of layer 3c which have been irradiated by dissolving in an appropriate solvent. This leads to a substrate 1 covered with layers 3a, 3b and a layer 3c, on which are inscribed the desired patterns, in the manner shown in FIG. 2.
At the end of this operation, layer 3b is etched by transferring thereto the patterns inscribed by etching in layer 3c, e.g. using trifluoromethane in the case where layer 3b is made from SiO.sub.2, which makes it possible to eliminate layer 3b at the points where it is not protected by the resin layer 3c. This leads to the structure shown in FIG. 3.
This is followed by the transfer of the etched patterns from layer 3b in the thick resin layer 3a by the action of a reactive gaseous plasma, such as an oxygen plasma, which makes it possible to obtain the structure shown in FIG. 4 and simultaneously eliminate the upper photosensitive resin layer 3c, which does not resist the action of the reactive gaseous plasma.
This procedure gives satisfactory results with regards to the resolution of the patterns, but suffers from the disadvantage of being difficult and costly to perform. Thus, the resin layers 3a and 3c can be deposited by centrifuging, but the intermediate layer 3b of SiO.sub.2, Si, Si.sub.3 N.sub.4 or Al is deposited by sputtering or chemical vapour phase deposition, which requires complicated costly equipment and more complex operations.